Transparent electrodes

ABSTRACT

A cathode adapted for use in an organic optoelectronic device is provided. The cathode has an electron injection layer, an organic buffer layer, a conducting layer, and a transparent conductive oxide layer disposed, in that order, over the organic operative layers of the optoelectronic device. A method of fabricating the cathode is also provided.

The subject matter of this application is related to concurrently filedpatent application Ser. No. 09/931,939, which is incorporated byreference in its entirety.

RESEARCH AGREEMENTS

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California and Universal Display Corporation. Theagreement was in effect on and before the date the claimed invention wasmade, and the claimed invention was made as a result of activitiesundertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to the field of organic semiconductordevices, and more particularly to transparent electrodes used in suchdevices.

BACKGROUND OF THE INVENTION

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials haveperformance advantages over conventional materials. For example, thewavelength at which an organic emissive layer emits light may generallybe readily tuned with appropriate dopants, while it may be moredifficult to tune inorganic emissive materials. As used herein, the term“organic material” includes polymers as wells as small molecule organicmaterials that may be used to fabricate organic opto-electronic devices.

OLEDs makes use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasingly populartechnology for applications such as flat panel displays, illumination,and backlighting. OLED configurations include double heterostructure,single heterostructure, and single layer, and a wide variety of organicmaterials may be used to fabricate OLEDs. Several OLED materials andconfigurations are described in U.S. Pat. No. 5,707,745, which isincorporated herein by reference in its entirety.

One or more transparent electrodes may be useful in an organicopto-electronic device. For example, OLED devices are generally intendedto emit light through at least one of the electrodes. For OLEDs fromwhich the light emission is only out of the bottom of the device, thatis, only through the substrate side of the device, a transparent anodematerial, such as indium tin oxide (ITO), may be used as the bottomelectrode. Since the top electrode of such a device does not need to betransparent, such a top electrode, which is typically a cathode, may becomprised of a thick and reflective metal layer having a high electricalconductivity. In contrast, for transparent or top-emitting OLEDs, atransparent cathode such as disclosed in U.S. Pat. Nos. 5,703,436 and5,707,745 may be used. As distinct from a bottom-emitting OLED, atop-emitting OLED is one which may have an opaque and/or reflectivesubstrate, such that light is produced only out of the top of the deviceand not through the substrate. In addition, a fully transparent OLEDthat may emit from both the top and the bottom.

The transparent cathodes as disclosed in U.S. Pat. Nos. 5,703,436 and5,707,745 typically comprise a thin layer of metal such as Mg:Ag with athickness, for example, that is less than about 100 angstroms. The Mg:Aglayer is coated with a transparent, electrically-conductive,sputter-deposited, ITO layer. Such cathodes may be referred to ascompound cathodes or as TOLED (“Transparent-OLED”) cathodes. Thethickness of the Mg:Ag and ITO layers in such compound cathodes may eachbe adjusted to produce the desired combination of both high opticaltransmission and high electrical conductivity.

The organic materials of an opto-electronic device may be verysensitive, and may be damaged by conventional semiconductor processing.For example, any exposure to high temperature or chemical processing maydamage the organic layers and adversely affect device reliability. Inaddition, exposure to air or moisture may also damage the sensitiveorganic layers. Such exposure may also damage any Mg:Ag layer that maybe present, because Mg:Ag is highly reactive. The organic materials mayalso be damaged by the processes used to deposit transparent electrodematerials. While conventional processes and structures allow for thefabrication of operational organic devices with transparent electrodes,the yield with such processes may be less than optimal. Indeed, theyield may not be great enough to fabricate commercially viable displays,for example, where the failure of only a few percent of the devices mayrender the display unsuitable for commercial use. There is therefore aneed for a method of fabrication and/or transparent electrode structuresthat result in higher yields.

SUMMARY OF THE INVENTION

A cathode adapted for use in an organic optoelectronic device isprovided. The cathode has an electron injection layer, an organic bufferlayer, a conducting layer, and a transparent conductive oxide layerdisposed, in that order, over the organic operative layers of theoptoelectronic device. A method of fabricating the cathode is alsoprovided.

A method of fabricating a device is provided. A substrate having firstconductive layer disposed thereon. An organic layer is fabricated overthe first conductive layer. A second conductive layer is then fabricatedover the organic layer such that the second conductive layer is inelectrical contact with the first conductive layer during at least aportion of the step of depositing the second conductive layer. Theelectrical contact between the first conductive layer and the secondconductive layer is then broken.

A method of fabricating an active matrix array of organic light emittingdevices is provided. A substrate is obtained, having circuitry adaptedto control the current flowing through each organic light emittingdevice, and having a first conductive layer disposed thereon, such thatthe first conductive layer is electrically attached to the circuitry. Anorganic layer is fabricated over the first conductive layer. A secondconductive layer is then fabricated over the organic layer such that thesecond conductive layer is in electrical contact with the circuitry, andsuch that the circuitry allows sufficient leakage between the firstconductive layer and the second conductive layer to reduce theelectrical field across the organic layer, during at least a portion ofthe step of fabricating the second conductive layer. The electricalcontact between the circuitry and the second conductive layer is thenbroken.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top-emitting organic light emitting device having atransparent multi-layer top electrode through which light may beemitted.

FIG. 2 shows a top-emitting organic light emitting device having atransparent multi-layer top electrode, having a structure different fromthat of FIG. 1, through which light may be emitted.

FIG. 3 shows a top-emitting organic light emitting device having atransparent multi-layer top electrode, having a structure different fromthat of FIGS. 1 and 2, through which light may be emitted.

FIG. 4 shows a top view of a first patterned conductive layer disposedover a substrate.

FIG. 5 shows a top view of a passive matrix display using firstpatterned conductive layer of FIG. 4.

FIG. 6 shows a circuit diagram of a single pixel of an active matrixOLED display

FIG. 7 shows a circuit diagram of 2×2 active matrix OLED array.

FIG. 8 shows a top schematic view of the array of FIG. 7.

FIG. 9 shows a cross section of FIG. 8 across line 9.

FIG. 10 shows a plot of current density v. voltage for two of theexample devices.

FIG. 11 shows normalized luminescence v. time for these devices.

DETAILED DESCRIPTION

In an embodiment of the invention, a multi-layer transparent cathodestructure is provided. The structure includes a layer of conductiveoxide, such as indium tin oxide (ITO), zinc indium tin oxide, oraluminum zinc oxide. The structure may also include an electroninjection layer that enhances the injection of electrons from thecathode into the organic layers of the device. The structure may alsoinclude an organic buffer layer that protects underlying organic layersduring the subsequent deposition of ITO. The structure may also includea conductive layer that enhances the injection of electrons from the ITOlayer into other layers of the electrode.

FIG. 1 shows a top-emitting organic light emitting device 100 having atransparent multi-layer top electrode 140 through which light may beemitted. Device 100 is disposed over substrate 110, and includes abottom electrode 120, an organic operative layer 130, and a topelectrode 140, deposited in that sequence over substrate 110. Topelectrode 140 further includes an electron injection layer 150, anorganic buffer layer 160, a conducting layer 170, and a transparentconductive oxide layer 180, deposited in that sequence over organicoperative layer 130.

Substrate 110 may be made of any conventional substrate material. Glass,plastic, metal foil, and ceramic substrates are known to the art.Substrate 110 may be rigid or flexible. Substrate 110 may be transparentor opaque. Substrate 110 may be a semiconductor such as silicon, whichmay facilitate the fabrication of circuitry connected to device 100 onnsubstrate 110.

Bottom electrode 120 may be made of any conventional electrode material.In the embodiment of FIG. 1, bottom electrode 120 is an anode. Indiumtin oxide (ITO) and metal anodes are known to the art. Bottom electrode120 may be transparent or opaque.

Organic operative layer 130 may be any organic layer known to the artthat may be incorporated into an opto-electronic device between theelectrodes of that device. In one embodiment, device 100 is an OLED, andoperative organic layer 130 may include any conventional OLED organicmaterials. The word “operative” is intended to distinguish organicoperative layer 130, i.e., the organic layer between the electrodes ofan optoelectronic device, from any organic layer that may be a part ofthe cathode, such as organic buffer layer 160, but is not intended tootherwise limit the layer. Organic operative layer 130 may be a singlelayer, or may include the multiple layers of conventional OLEDstructures, such as a single or double heterostructure, or one or morelayers containing a mixture of OLED organic materials. In otherembodiments, organic operative layer 120 may be the one or more organiclayers of a photodetector, photovoltaic cell, phototransistor, or otheropto-electronic device.

Top electrode 140 further includes an electron injection layer 150, anorganic buffer layer 160, a conducting layer 170, and a transparentconductive oxide layer 180, deposited in that sequence over organicoperative layer 130. In one embodiment, it is contemplated that topelectrode 140 may also include other layers. In such an embodiment, eachlayer may be described as “disposed over” an underlying layer. Forexample, organic buffer layer 160 may be described as “disposed over”electron injection layer 150 to allow for the possibility that there isone or more additional layers deposited between organic buffer layer 160and electron injection layer 150. In another embodiment, it iscontemplated that there are no layers other than those described, inwhich case the layers may be described as “disposed over and in physicalcontact with” the underlying layer. However, even in this embodiment itis contemplated that interfacial effects may occur between the layersdescribed.

Electron injection layer 150 may improve the injection of electrons fromorganic buffer layer 160 into organic operative layer 130. A preferredelectron injection layer 150 further comprising two layers, a layer ofLiF disposed over organic operative layer 130, and a layer of Aldisposed over the layer of LiF. Another preferred electron injectionlayer material is LiF co-evaporated with Al. Other preferred electroninjection layer materials include Li₂O, CsF, alkali metal halides, andalkaline earth metal halides. The electron injection layer thickness ispreferably as thin as possible, while still retaining electron injectioncapabilities. For example, the thickness is preferably about 5-25 Å, andmore preferably about 25 Å. At these thicknesses, it is possible thatthe layers are not contiguous, and may form islands. It is also possiblethat much of the material, for example LiF, may dissociate and that theresultant Li may diffuse into other layers, such as organic buffer layer160. Even if the layers are not contiguous and if there is significantdissociation and diffusion, the term “layer” is intended to describe theresult of depositing the electron injection material.

Organic buffer layer 160 may reduce damage to underlying organicoperative layer 130 during the fabrication of subsequently depositedlayers. In particular, the processes used to deposit transparentconductive oxide layer 180 may damage organic operative layer in theabsence of a buffer layer. Preferably, organic buffer layer has a highelectron mobility, such that it does not significantly increase theoperating voltage of device 100. Preferably, the electron mobility oforganic buffer layer 160 is higher than that of any electron transportlayer that may be included in organic operative layer 130. Preferably,organic buffer layer 160 does not increase the operating voltage ofdevice 100 more than about 25% relative to a similar device with noorganic buffer layer. More preferably, this increase is not more thanabout 10%. Preferred materials for organic buffer layer 160 includecopper phthalocyanine (CuPc), and2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP). Other preferredmaterials include various metal phthalocyanines, such as ZnPc, MgPc, andPbPc. The thickness of organic buffer layer 160 is preferably thickenough that there is little or no damage to organic operative layer 130due to fabrication processes that occur after organic buffer layer 160is deposited, yet not so thick as to significantly increase theoperating voltage of device 100. Preferably, buffer layer 160 is about50-500 Å thick, and more preferably about 400 Å thick. Buffer layer 160may be doped to increase its conductivity. For example, a CuPc or BCPbuffer layer 160 may be doped with Li.

Conductive layer 170 may improve electron injection from transparentconductive oxide layer 180 into organic buffer layer 160. The injectionof electrons from some transparent conductive oxides, such as ITO, intosome organic buffer layers, such as CuPc, may be less than optimal.Conductive layer 170 may improve such injection. Preferred materials forconductive layer 170 include Mg:Ag, other metals, Ca, and LiF/Al, andany materials described above as preferred materials for electroninjection layer 150. Conductive layer 170 is preferably very thin,because very little thickness is needed to improve electron injection,and any additional thickness may reduce the transparency of device 100.Where Mg:Ag is used, the thickness may preferably be up to about 125 Å,and more preferably about 100 Å or less. Where Mg:Ag is used, the ratioof Mg to Ag is preferable about 10:1 to 30:1. Where LiF/Al is used,conductive layer 170 is preferably similar to the LiF/Al layer describedabove as a potential electron injection layer 150.

Transparent conductive oxide layer 180 may be responsible for most ofthe conductivity of electrode 140 in the plane parallel to substrate110. The material and thickness of transparent conductive oxide layer180 may be chosen to achieve a desired combination of conductivity andtransparency. Preferred materials for transparent conductive oxide layerinclude ITO, zinc indium tin oxide, and aluminum zinc oxide. Preferably,transparent conductive oxide layer is about 100-2000 Å thick, and morepreferably about 500-1000 Å thick.

The materials and thicknesses of each of the layers of top electrode 140may be selected in combination to control the optical properties of topelectrode 140. For example, the materials and thicknesses may beselected to obtain a desired transparency. For applications where a hightransparency is desirable, a transparency of greater than about 90% maybe obtained. For other applications, a lower transparency may bedesirable, and such transparencies may also be readily achieved throughselection of materials and thicknesses. For example, a transparency ofabout 30% may be satisfactory for certain applications, and an electrodehaving such a transparency is considered “transparent” for suchapplications. Other optical properties may also be controlled bymaterial selection and dopants. For example, CuPc may act as a bluefilter, reducing the amplitude of visible light in the red region.Various dopants may be added to the layers of top electrode 140 tomodify their optical properties.

Top electrode 140 may be used as the cathode of a device 100 that emitslight through top electrode 140, but not through bottom electrode 120and substrate 110. In such a device, the materials and thicknesses ofbottom electrode 120 and substrate 110 are preferably selected tomaximize back reflection, for example, a thick, highly reflective metallayer. Top electrode 140 may also be used as the cathode of a device 100that emits light through both top electrode 140 and bottom electrode120. In such a device, the materials and thicknesses of all layers maybe selected to achieve a desired level of transparency.

FIG. 2 shows a top-emitting organic light emitting device 200 having atransparent multi-layer top electrode 240 through which light may beemitted. Device 200 is disposed over substrate 210, and includes abottom electrode 220, an organic operative layer 230, and a topelectrode 240, deposited in that sequence over substrate 210. Topelectrode 240 further includes an electron injection layer 250, aconducting layer 270, and a transparent conductive oxide layer 280,deposited in that sequence over organic operative layer 230.

The materials and thicknesses of substrate 210, bottom electrode 220,organic operative layer 230, electron injection layer 250, conductinglayer 270, and transparent conductive oxide layer 280 of device 200 maybe similar to those of substrate 110, bottom electrode 120, organicoperative layer 130, electron injection layer 150, conducting layer 170,and transparent conductive oxide layer 180, respectively, of device 100.

The embodiment of FIG. 2 involves a trade-off when compared to that ofFIG. 1. The absence of a buffer layer results in a simpler structure,with less manufacturing steps. However, the process used to deposit theconductive oxide layer, such as ITO, may be more restricted due to theabsence of the protective buffer layer and the need to minimize damageto the operative organic layers. For example, a lower ITO depositionrate and different partial pressures of the various gases involved maybe used in the embodiment of FIG. 2 as compared to the embodiment ofFIG. 1.

FIG. 3 shows a top-emitting organic light emitting device 300 having atransparent multi-layer top electrode 340 through which light may beemitted. Device 300 is disposed over substrate 310, and includes abottom electrode 320, an organic operative layer 330, and a topelectrode 340, deposited in that sequence over substrate 310. Topelectrode 340 further includes an electron injection layer 350, a bufferlayer 360, and a transparent conductive oxide layer 380, deposited inthat sequence over organic operative layer 330.

The materials and thicknesses of substrate 310, bottom electrode 320,organic operative layer 330, electron injection layer 350, buffer layer360, and transparent conductive oxide layer 380 of device 300 may besimilar to those of substrate 110, bottom electrode 120, organicoperative layer 130, electron injection layer 150, buffer layer 160, andtransparent conductive oxide layer 180, respectively, of device 100.

The embodiment of FIG. 3 involves a trade-off when compared to that ofFIG. 1. The absence of a conductive layer results in a simplerstructure, with less manufacturing steps. Also, the absence of aconductive layer may result in an electrode having a highertransparency. However, while not intending to be limited as to anytheory of how the invention works, it is believed that there is adifference between the embodiments of FIGS. 1 and 3 in the mechanism bywhich electrons are injected from the conductive oxide into the bufferlayer, which may result in a slightly higher operating voltage for theembodiment of FIG. 3. In particular, it is believed that the electroninjection from the conductive oxide into the buffer layer in theembodiment of FIG. 3 is facilitated by damage states in the bufferlayer. This mechanism is described in more detail in U.S. Pat. No.6,420,031, issued to Parthasarathy et al. entitled “Highly TransparentNon-Metallic Cathodes,” which is incorporated by reference in itsentirety. In addition, the cathode of FIG. 3 may be slightly lessconductive than that of FIG. 1 in lateral directions, i.e., directionsparallel to the substrate.

Top electrodes 140, 240 and 340 of FIGS. 1, 2 and 3 may advantageouslybe used as the cathode of an opto-electronic device, because of theelectron injection layers included therein. By replacing the electroninjection layer with an appropriate hole injection layer, the structuresmay advantageously be used as the anode of an opto-electronic device.Top electrodes 140 and 340 may advantageously be used as top electrodes,because of the buffer layer included therein. The structure of topelectrode 240 may be advantageously used as a top electrode (cathode) oras a bottom electrode (cathode).

In addition, the organic materials of organic opto-electronic devicesmay sustain “charging damage” during fabrication. Charging damage is theresult of a voltage applied across the device during fabrication,generally due to secondary electrons generated during the processes usedto deposit electrodes after the organic layers are already present.Charging damage may occur during e-beam processes or plasma relatedprocesses, such as sputtering, plasma-enhanced chemical vapordeposition, and reactive ion etching. Unfortunately, many of the mostdesirable transparent electrode materials, such as indium tin oxide(ITO), are generally deposited using processes such as sputtering thatgenerate secondary electrons. Again, conventional methods may beutilized to fabricate operational devices, but the yield may be lessthan optimal.

In an embodiment of the invention, a method of fabricating anopto-electronic device is provided that reduces charging damage. After afirst electrode and an organic layer are fabricated, a second electrodeis fabricated such that the second electrode is in electrical contactwith the first electrode during at least a portion of the step ofdepositing the second electrode. This electrical contact may be througha direct conductive path, or it may be indirect, through circuitry thatallows for enough leakage to reduce charging damage. Because the firstand second electrode are in electrical contact during the fabrication ofthe second electrode, a significant voltage difference between the firstand second electrodes is not sustainable, and charging damage isreduced. In order to create a functional device, the electrical contactbetween the first and second electrodes is broken after the secondelectrode is fabricated.

In one embodiment, the second electrode may be electrically connected toground instead of to the first electrode during at least a portion ofthe step of depositing the second electrode. However, this embodiment isless preferable than electrically connecting the second electrode to thefirst electrode, because there may be a voltage difference betweenground and the first electrode, which may have floating voltage. Evenso, this embodiment may reduce the voltage difference between the firstand second electrodes during fabrication of the second electrode. Aconnection to “ground” may be established, for example, by an electricalconnection to a conductive wall of the chamber in which the device isbeing fabricated.

FIG. 4 shows a first patterned conductive layer 420 disposed over asubstrate 410. The particular pattern shown in FIG. 4 is designed foruse in a passive matrix display. The materials and thicknesses ofsubstrate 410 and first patterned conductive layer 420 may be anysuitable substrate and electrode materials, respectively. Indium tinoxide (ITO) is a preferred material for first patterned conductive layer420.

FIG. 5 shows a passive matrix display using first patterned conductivelayer 420 of FIG. 4, which is disposed over substrate 410. Firstpatterned conductive layer 420 includes three first electrodes 525, aswell as residual portions 523. Scribe lines 550 define the boundarybetween first electrodes 525 and residual portions 523. An organic layer530 is disposed over first electrode 525. A second patterned conductivelayer 540 is disposed over organic layer 530. Second patternedconductive layer 540 includes three second electrodes 545, as well asresidual portions 543. Scribe lines 550 define the boundary betweensecond electrodes 545 and residual portions 543. Scribe lines 550 alsodefine the boundary between a portion 515 of substrate 410, and residualportions 513 of substrate 410.

Organic layer 530 may comprise any suitable organic layer. Organic layer530 may also comprise multiple organic layers, such as the multiplelayers of a double heterostructure or single heterostructure OLED.Organic layer 530 may be the operative layer of any organicoptoelectronic device, including an OLED, a photovoltaic cell, aphotodetector, a phototransistor, etc.

Although organic layer 530 is shown as patterned into strips parallel tosecond electrode 540, other configurations may be used. For example,organic layer 530 may be a blanket layer, or organic layer 530 may bepatterned into strips parallel to first electrode 420, or otherconfigurations may be used.

The array of FIG. 5 may be fabricated as follows. A substrate 410 havingthereon a first patterned conductive layer 420 is obtained or fabricatedusing any suitable technique, including techniques known to the art.Organic layer 530 is fabricated over first patterned conductive layer420 using any suitable technique, including techniques known to the art.Second patterned conductive layer 540 is fabricated over organic layer530 using any suitable technique, including techniques known to the art,such that there is electrical contact between first patterned conductivelayer 420 and second patterned conductive layer 540 over residualportions 513 of substrate 410 during at least a portion of the step ofdepositing second patterned conductive layer 540. The electricalconnection between first electrode 525 and second electrode 545 is thenbroken, for example by scribing and breaking substrate 410 along scribelines 550, such that residual portions 513 are removed along with anylayers deposited thereon.

The electrical contact between first patterned conductive layer 420 andsecond patterned conductive layer 540 advantageously reduces “chargingdamage” during the deposition of second patterned conductive layer 540that might otherwise occur. The terminology “during at least a portion”is used to indicate that electrical contact may not yet be formed at thevery beginning of the step. For example, if second patterned conductivelayer is a single layer of ITO, the very first particles deposited maynot be in electrical contact with each other, let alone first patternedconductive layer 420. However, at some point, second patternedconductive layer 540 becomes sufficiently contiguous that is may beconsidered in electrical contact with first patterned conductive layer420. Preferably, second patterned layer 540 is in electrical contactwith first patterned conductive layer 420 substantially throughout thedeposition process.

In a preferred embodiment, second patterned conductive layer 540, andsecond electrode 545, are multi-layer structures that include a layer ofmetal deposited over organic layer 530, and a layer of ITO depositedover the layer of metal. Using conventional fabrication techniques, itis believed that most charging damage occurs during the deposition ofITO, which is generally deposited using a technique such as sputteringthat involves secondary electrons. In this preferred embodiment, secondpatterned conductive layer 540 is in electrical contact with firstpatterned conductive layer 420 via the layer of metal during the entiredeposition of the ITO. The layer of metal may be deposited usingtechniques such as subliminal evaporation that do not tend to causecharging damage.

In a preferred embodiment, the multilayer top electrode of FIG. 1, 2 or3 is used as top electrode 540 of FIG. 5.

The electrical contact between first patterned conductive layer 420 andsecond patterned conductive layer 540 during at least a portion of thestep of fabricating second patterned conductive layer 540 may beachieved in a number of different ways. One is by direct physicalcontact between the two layers, as illustrated in FIG. 5.

Another is by contact through a conductive medium, such as a conductivepaste or tape present on residual portions 513 of substrate 410. In thisalternative, the parts of patterned conductive layers 420 and 540extending onto residual portions 513 of substrate 410, i.e., residualportions 523 and 543, may be much smaller than depicted in FIG. 5. Anyother suitable means of achieving this electrical contact may also beused.

The breaking of electrical contact between first electrode 525 andsecond electrode 545 may also be achieved in a number of ways. One is byscribing and breaking substrate 410 along scribe lines 550, asillustrated in FIG. 5.

Another way of breaking the electrical contact between first electrodes525 and second electrodes 545 is through a subtractive etching process,during which the region of substrate 410 occupied by first electrodes525 and second electrodes 545 is protected by a mask or a passivationlayer selectively deposited prior to the subtractive etching process.Suitable subtractive processes include reactive ion etching and oxygenplasma etching.

Another way of breaking the electrical contact between first electrodes525 and second electrode 545 is lift-off. In particular, a photoresistor other material that may be readily and selectively removed may bedeposited onto residual portions 513 of substrate 410 prior to thefabrication of first electrodes 525. After second electrodes 545 havebeen fabricated, the photoresist may be removed, along with any layersdeposited thereon, breaking the electrical contact.

Another way of breaking the electrical contact between first electrodes525 and second electrodes 545 is by using a conductive paste, tape orfilm to achieve the contact, and then removing the conductive paste,tape or film by wiping, peeling, or a similar method. Any other suitablemethod of breaking the electrical contact may be used, in addition tothose specifically described herein.

FIG. 6 shows a circuit diagram of a single pixel of an active matrixOLED display. Organic light emitting device 610 has an anode 620 and acathode 630. Power is provided though power line 650 (Vdd). Cathode 630is connected to ground, or a fixed reference voltage, though line 660.The voltage at anode 620 is determined by control circuitry 640 based oninput from data lines 670 and 680. Control circuitry 640 may includevarious configurations for controlling a pixel, including those known tothe art, and is not limited to the specific circuit illustrated in FIG.6. For example, two transistors in series may be disposed between powerline Vdd and anode 620.

FIG. 7 shows a circuit diagram of 2×2 active matrix OLED array. Eachpixel 710 of the array may further include the circuit diagram of FIG.6.

FIG. 8 shows a top schematic view of the array of FIG. 7. FIG. 9 shows across section of FIG. 8 across line 9. The array is fabricated on asubstrate 605. Control circuitry 640 (not shown) may be on substrate605. Preferably, substrate 605 may be made of a material similar tothose described for substrate 110 of FIG. 1. Anode 620 is disposed oversubstrate 605, and may be on substrate 605, with an exposed surface, asillustrated. An organic layer 625 (only shown in FIG. 9, not shown inFIG. 8) is disposed over anode 620. Cathode 630, a large conductivesheet spanning the entire array, is disposed over organic layer 605. Thevoltage at each anode 610 may be individually controlled by signalsapplied through data lines 670 and 680 (not shown in FIGS. 8 and 9).Power line 650 and data lines 670 and 680 may be on substrate 605, withexposed contacts as illustrated in FIG. 8.

The embodiment of FIGS. 6 through 9 may be fabricated as follows.Substrate 605, control circuitry 640, power lines 650, data lines 670and 680, and anodes 620 may be fabricated using any suitable technique,including techniques known to the art. Organic layer 625 is thendeposited over anodes 620. Cathode 630 is then deposited over organiclayer 625, such that cathode 630 is in electrical contact with powerlines 650 during at least a portion of the step of depositing cathode630. This electrical contact is preferably implemented by electricallyconnecting cathode 630 to the exposed contact of power lines 650 asillustrated in FIG. 9. Any of the techniques described with respect tothe embodiment of FIGS. 4 and 5 may be used to achieve this electricalcontact. Although power line 650 is not necessarily in direct electricalcontact with anodes 610 during the deposition of cathode 630, controlcircuitry 640 should allow sufficient leakage, such that the fieldstrength between anode 620 and cathode 630 should not exceed about 106V/cm, corresponding to a voltage of about 10 V across a 1000 Å thickdevice, during the deposition of cathode 630 using most conventionaldeposition methods for conductive oxide materials.

This leakage may be obtained in several ways. The characteristics of thetransistors, generally thin film transistors, may such that the leakageoccurs. For example, for an average pixel size, the capacitance is lessthan about 10 pF. A field of 10⁶ V/cm corresponds to about 10 V across a1000 Å device. A rough estimate of sufficient leakage is that thevoltage discharges in about 1 second, which is much less than anexpected deposition time of about 100 seconds. Using the equationC*V=I*T, where C=10⁻¹¹, V=10, and T=1, the result is I=10⁻¹⁰ A. If theleakage of the transistors is insufficient, several options exist.First, the transistor controlling the OLED may be designed to increaseits leakage, for example by reducing the channel length, or adjustingthe threshold voltage to increase leakage when no bias is applied.Although leakage in transistors is generally viewed as undesirable, thereduction in charging damage may justify a higher leakage for thisapplication. Second, an electrical bias may be applied to the controlcircuitry, for example through data lines 670 and 680, during thedeposition of the second conductive layer such that the transistorconducts.

Preferred materials and thicknesses of organic layer 625 and cathode 630are similar to those for the embodiment of FIGS. 4 and 5.

Preferably, the embodiment of FIGS. 6 through 9 uses as a cathode one ofthe embodiments described in FIGS. 1 through 5.

It is to be understood that the present invention may be used tofabricate much larger arrays of organic devices than those specificallydescribed herein. For example, although FIG. 5 shows a 3×3 array ofdevices, and FIG. 7 shows a 2×2 array, much larger arrays may befabricated.

Although many of the embodiments are not specifically described withrespect to multicolor displays, it is understood that all embodimentsmay be readily adapted for use in a multicolor display by one ofordinary skill in the art. For example, a multi-color display may befabricated by depositing various down-conversion layers known to theart, or using different organic materials in different devices. Amulti-color array may also be fabricated by a number of other methods,such as using an array of white-emitting OLEDs in combination with colorfilters or a distributed Bragg reflector.

It is also to be understood that the present invention is not limited tothe specific illustrated embodiments, and may be used to in a widevariety of other embodiments.

The present invention may be used to fabricate a number of productsincorporating organic opto-electronic devices, including flat paneldisplays, organic photodetectors, organic phototransistors, organicphotovoltaic cells, and organic photodiodes, both in arrays and asindividual devices.

EXAMPLES

The following structures were fabricated on a 6″ OLED deposition toolhaving an island cluster tool, obtained from the Kurt Lesker company ofPittsburgh, Pa. Each structure was fabricated on a Rel-4-Regularsubstrate, which is an ITO coated soda lime glass obtained from AppliedFilms Corp. of Colorado. Prior to fabricating each structure, thesubstrate was preheated to 100 degrees C. for 2 minutes, and thensubjected to an oxygen plasma treatment, 50 W, 100 mTorr for 2 minutes.

Example 1

For comparative purposes, an array of OLEDs similar to the prior art wasfabricated by depositing the following layers, in sequence, throughappropriate masks. The deposition of all material was by thermalevaporation, except for ITO (which was only deposited in examples 2, 3and 4), which was deposited by sputtering. Due to the use of differentfeedback mechanisms to monitor the deposition sources, the sourceparameter that was controlled may be either a temperature or a voltage.The MgAg layer was co-deposited from separate Mg and Ag targets to thetotal thickness shown (1000 Å in example 1).

Material Dep. Rate (Å/s) Thickness Source Temperature (C.) or Power CuPc1.2 100 345.1 C. NPD 3 500 254.4 C. Alq 3 500 266.7 C. LiF 1-2 14 41.2 VAl 2 10 88.9 V CuPc 2 400 387.4 C. MgAg   3 (Mg) 1000 40.5 V 1.2 (Ag) —43.5 V

Although an Ag deposition rate of 1.2 was read on the dial, it isexpected that the deposition rate was actually about 0.3 due to atooling factor in this and in the other examples.

Example 2

An array of OLEDs similar to the device shown in FIG. 1 was fabricatedby depositing the following layers, in sequence. The equipment and masksused were the same as for example 1:

Material Dep. Rate (Å/s) Thickness Source Temperature (C.)/Power CuPc1.2 100 345.1 C. NPD 3 500 257-258 C. Alq 3 500 304-331 C. LiF 1-2 1440.9-41.4 V Al 2 10 88.9 V CuPc 2 400 347-396 C. MgAg   3 (Mg) 100 41.5V 1.2 (Ag) — 43.5 V

A layer of ITO was then deposited by sputtering. The chamber contained 3mTorr of an Ar/O₂ mixture, and the power applied to the chamber wasabout 250 W. The power applied to the target was about 1.25-1.26 A at1.98.6-197 V. The substrate was passed over the target 3 times at aspeed of 17 inches per minute. With these parameters, it is expectedthat the thickness of ITO deposited was about 500 Å.

Example 3

An array of OLEDs similar to the device shown in FIG. 3 was fabricatedby depositing the following layers, in sequence. The equipment and masksused were the same as for example 1:

Material Dep. Rate (Å/s) Thickness Source Temperature (C.)/Power CuPc1.2 100 345.1 C. NPD 3 500 249 C. Alq 3 500 263 C. LiF 1-2 14 40.0-40.7V Al 2 10 88.9 V CuPc 2 400 348 C.

A layer of ITO about 500 Å thick was deposited by sputtering usingparameters similar to those given for example 2, and the power appliedto the target was 1.26 A at 198.3-197.5 V.

Example 4

For comparative purposes, an array of OLEDs was fabricated by depositingthe following layers, in sequence. The equipment and masks used were thesame as for example 1:

Material Dep. Rate (Å/s) Thickness Source Temperature (C.)/Power CuPc1.2 100 336.2 C. NPD 3 500 257.4 C. Alq 3 500 257.8 C. MgAg   3 (Mg) 10038.8 V 1.2 (Ag) — 43.7 V

A layer of ITO about 500 Å thick was deposited by sputtering usingparameters similar to those given for example 2, and the power appliedto the target was 1.26 A at 197.1-197.2 V.

FIG. 10 shows a plot of current density v. voltage for two of theexample devices. Plot 1010, illustrated by diamonds, shows the currentdensity v. voltage for a device fabricated as described in example 2.Plot 1020, illustrated by squares, shows the current density v. voltagefor a device fabricated as described in example 1. The plots show thatthe device of example 2 has a significantly higher current density thanthe device of example 1.

Without intending to be limited as to any theory as to how the inventionworks, it is believed that the ITO deposition process may knock some Mgfrom the previously deposited Mg:Ag layer into the CuPc layer, therebyimproving electron injection and lowering the operating voltage.

Lifetime tests were performed on devices fabricated per examples 2 and4. The devices were run with a constant current of 10 mA/cm2 for about1400 hours. FIG. 11 shows normalized luminescence v. time for thesedevices. Plot 1110 shows the normalized luminescence v. time for adevice fabricated according to example 4, illustrated by dark squares.Plot 1120 shows the normalized luminescence v. time for a devicefabricated according to example 2, illustrated by triangles. The plotsshow that the device of example 2 retains its ability over time toluminesce much better than the device of example 4.

Without intending to be limited to any theory as to why the inventionworks, it is believed that the improved electron injection of thedevices of Example 2 leads to a better balance of carriers in the OLED,which improves the OLED lifetime.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. Inparticular, the present invention is not limited to OLEDs, and may beapplied to a wide variety of opto-electronic devices. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. An optoelectronic device, comprising: (a) an electron injection layerdisposed over an organic operative layer of the optoelectronic device;(b) an organic buffer layer disposed over the electron injection layer;(c) a conducting layer disposed over the organic buffer layer; (d) atransparent conductive oxide layer disposed over the conducting layer.2. The device of claim 1, wherein the device is an organic lightemitting device, and the electron injection layer comprises a layer ofLiF adapted to be electrically connected to an organic emissive layer ofthe device, and a layer of Al electrically connected to the layer ofLiF.
 3. The device of claim 1, wherein the electron injection layerfurther comprises a layer of Li₂O.
 4. The device of claim 1, wherein theelectron injection layer further comprises a layer of CsF.
 5. The deviceof claim 1, wherein the electron injection layer further comprises alayer of alkali metal halide.
 6. The device of claim 1, wherein theelectron injection layer further comprises a layer of alkaline earthmetal halide.
 7. The device of claim 1, wherein the buffer layer furthercomprises a layer of CuPc.
 8. The device of claim 7, wherein the bufferlayer further comprises a layer of CuPc doped with Li.
 9. The device ofclaim 1, wherein the buffer layer further comprises a layer of BCP. 10.The device of claim 9, wherein the buffer layer further comprises alayer of BCP doped with Li.
 11. The device of claim 1, wherein theconducting layer further comprises a layer of MgAg.
 12. The device ofclaim 1, wherein the conducting layer further comprises a layer of Ca.13. The device of claim 1, wherein the conducting layer furthercomprises a layer of LiF and a layer of Al.
 14. The device of claim 1,wherein the transparent conductive oxide layer further comprises a layerof indium tin oxide.
 15. The device of claim 1, wherein the transparentconductive oxide layer further comprises a layer of zinc indium tinoxide.
 16. The device of claim 1, wherein the transparent conductiveoxide layer further comprises a layer of aluminum zinc oxide.
 17. Thedevice of claim 1, wherein, the electron injection layer, the organicbuffer layer, the conducting layer and the transparent conductive oxidelayer in combination have transparency of at least 90%.
 18. The deviceof claim 1, wherein the buffer layer does not increase the operatingvoltage of the device more than about 25% compared to a device having nobuffer layer.
 19. The device of claim 18, wherein the buffer layer doesnot increase the operating voltage of the device more than about 10%compared to a device having no buffer layer.
 20. The device of claim 1,wherein the optoelectronic device is an organic light emitting devicehaving an electron transport layer, and wherein the buffer layer has anelectron mobility higher than that of the electron transport layer. 21.The device of claim 1, wherein the optoelectronic device is an organiclight emitting device.
 22. The device of claim 1, wherein: (a) theelectron injection layer is in physical contact with the organicoperative layer of the optoelectronic device; (b) the organic bufferlayer is in physical contact with the electron injection layer; (c) theconducting layer is in physical contact with the organic buffer layer;(d) the transparent conductive oxide layer is in physical contact withthe conducting layer.
 23. An organic light emitting device, comprising:(a) a layer of LiF adapted to be physically and electrically connectedto an organic operative layer of the organic light emitting device; (b)a layer of Al physically and electrically connected to the layer of LiF;(c) a layer of CuPc physically and electrically connected to the layerof Al; (d) a layer of MgAg physically and electrically connected to thelayer of CuPc; (e) a layer of ITO physically and electrically connectedto the layer of CuPc.
 24. A device, comprising: (a) an anode disposedover a substrate; (b) an organic operative layer disposed over andelectrically connected to the anode; (c) an electron injection layerphysically and electrically connected to the organic operative layer;(d) an organic buffer layer physically and electrically connected to theelectron injection layer; (e) a conductive layer physically andelectrically connected to the organic buffer layer; (f) a transparentconductive oxide layer physically and electrically connected to theconductive layer.
 25. A method of fabricating an optoelectronic devicehaving a transparent cathode, comprising the steps of: (a) depositing anelectron injection layer over an organic operative layer of the organicoptoelectronic device; (b) depositing an organic buffer layer over theelectron injection layer; (c) depositing a conductive layer over theorganic buffer layer; (d) depositing a transparent conductive oxidelayer over the conductive layer.
 26. The method of claim 25, wherein theelectron injection layer includes LiF and Al.
 27. The method of claim25, wherein the electron injection layer includes Li₂O.
 28. The methodof claim 25, wherein the electron injection layer includes CsF.
 29. Themethod of claim 25, wherein the electron injection layer includes analkali metal halide.
 30. The method of claim 25, wherein the electroninjection layer includes an alkaline earth metal halide.
 31. The methodof claim 25, wherein the buffer layer includes CuPc.
 32. The method ofclaim 31, wherein the buffer layer further includes Li.
 33. The methodof claim 25, wherein the buffer layer includes BCP.
 34. The method ofclaim 33, wherein the buffer layer further inludes Li.
 35. The method ofclaim 25, wherein the conducting layer includes Mg and Ag.
 36. Themethod of claim 25, wherein the conducting layer includes Ca.
 37. Themethod of claim 25, wherein the conducting layer includes LiF and Al.38. The method of claim 25, wherein the conductive oxide layer includesindium tin oxide.
 39. The method of claim 25, wherein the conductiveoxide layer includes zinc indium tin oxide.
 40. The method of claim 25,wherein the conductive oxide layer includes aluminum zinc oxide.
 41. Anorganic optoelectronic device having a transparent cathode fabricated bythe steps of: (a) depositing an electron injection layer over an organicoperative layer of the organic optoelectronic device; (b) depositing anorganic buffer layer over the electron injection layer; (c) depositing aconductive layer over the organic buffer layer; (d) depositing atransparent conductive oxide layer over the conductive layer.